Some observations on the antigenic structure of trichloroacetic acid extracts in Escherichia and the Paracolon group.

The term‘ paracolon ’ is passing out of common use and is applied here descriptively rather than definitively. Some strains were encountered which were members of the family Enterobacteriaceae and resembled coliform organisms in their morphological and cultural characteristics ; they were late, or non-lactosefermenters, and did not belong to Escherichia, Providence, Bethesda-Ballerup or other similar groups. Interest in the group was stimulated by an unusual case which has been reported elsewhere (Macpherson & Exner-Baumann, 1950). At this time, investigators were pessimistic about the possibility of classifying the paracolon group by conventional serological methods (e.g. Stuart, Galton & McGann, 1948; Mushin, 1949). The use of trichloroacetic acid (TCA) extracts as suggested by Boivin, Mesrobeanu, Magheru & Magheru (1935) was considered. The success which had attended their employment in Pseudomonas (van den Ende, 1952) was felt to be encouraging and the technique was accordingly adopted. Some success was obtained and the serological classification of biochemically homogeneous groups was possible. This classification was, of course, based on the somatic antigenic structure of the strains and could be confirmed and amplified by the use of ‘0’ and ‘H’ antisera (Macpherson, 1954). Anomalies were encountered, however, and it appeared that they were due either to the failure of the TCA to extract certain somatic antigens, or to the presence of antigens of ‘ envelope ’ type. To confirm either of these possibilities, i t was necessary to employ strains which shared labelled somatic or envelope antigens. By preparing antisera against such strains and then studying the cross-reactions obtained with TCA extracts, it should be evident whether some, all or no such antigens were being extracted. Since suitable paracolon strains were not available, Escherichia strains were obtained from Dr F. Kauffmann of Copenhagen. It has been emphasized, almost since antigenic extracts were first studied, that they are not chemically homogeneous, and the occurrence of degradation